Referring to FIG. 1 which shows a construction known in the art, therein it is seen that the valve seat-forming member of a fuel injection valve of the described type includes a guide bore b for the guiding of opening and closing displacement movements of the valve rod, a valve seat c formed at the end of the guide bore b in continuation from the guide bore, and a fuel discharge port d in continuation from the valve seat c. In this construction, it is the customary practice to polish a bore b' bored in a blank of a valve seat forming member a, and to polish a bore c' in the valve seat c in order to form the guide bore b and a seat surface e of the valve seat c. In greater detail, the bore b' is polished by a grinding tool f that is moved around an axis of rotation l.sub.1 aligned with the axis of the guide bore b, while, rotating the tool about its own axis of rotation l.sub.2 which is parallel to the axis of rotation l.sub.1. The seat surface e of the valve seat c is polished by a grinding tool which has a different shape from that of the grinding tool f.
This method produces the following problems:
1. The polishing finish requires a relatively large thickness or margin D for polishing. Therefore if an error, even slight, occurs in the mounting position of the grinding tool f, for example, due to displacement in the position of the axis of rotation l.sub.2 from the axis of rotation l.sub.1, the guide bore b can not be polished and finished uniformly.
2. The tip of the grinding tool f is easily worn so that it can produce a sharp flashing or edge g at the gude bore b i.e. at the juncture of the guide bore b and the seat c. The flashing g has a width corresponding to the extent of the polishing margin D. In order to prevent the formation of the flashing or edge g, an undercut escape groove h must be formed in advance by cutting into the end of the bore b'; however, this is difficult to do.
3. The escape groove h forms, in turn, a fuel trap within the valve seat-forming member a causing a drop in the fuel velocity which has an adverse influence on the fuel injection.
4. In connection with problem (2) above, if the swarf produced when cutting the escape groove h remains, it can choke the fuel discharge port d or cause fuel leakage. To remove the swarf, thorough cleaning must be effected, but it is extremely difficult to completely remove the swarf. This leads to marked variations in the fuel injection quantity and performance.